A traditional sleeping bag typically comprises an elongated bag comprised of an insulated blanket. It has an open top end and a closed bottom end and often includes a zipper or other closure means extending along at least one elongated edge thereof for opening and closing the bag. A user typically positions himself in the bag between top and bottom sides while the zipper is at least partially unfastened and then closes the bag to substantially encapsulate his body within the bag with his head often extending out of the open top end.
Sleeping bags are typically used by campers and backpackers to provide warmth when sleeping outside. Sleeping bags are often assigned temperature ratings which indicate the outside temperature at which a user can sleep in bag either comfortably or without undue risk of hypothermia. As can be appreciated, the temperature rating of a particular bag is directly affected by the amount of insulation or “fill” provide in the insulated blanket that comprises the sleeping bag. Simply, more fill allows the bag to be used at lower temperature while less fill results in a bag that permits a more comfortable sleep at higher outside temperatures. Traditional fill materials include down and synthetic batting. The loft or thickness of the fill material creates pockets of dead air that hinder the transfer of heat from the user's body thereby keeping him/her warm.
When a user lies in the bag, the fill underneath him and in contact with the underlying surface compresses, eliminating many of the air pockets and thereby reducing the insulating capacity of the bag in that region. Accordingly, campers and backpackers that are going to be sleeping on an uninsulated surface such as the ground typically utilize a sleeping pad. Sleeping pads typically comprise a firm flexible foam material that does not compress completely when laid upon, thereby sufficiently insulating a user from the cold ground.
Considering the foregoing, it can be appreciated that the primary function of the compressed portions of the sleeping bag located between a sleeping user and an underlying surface is not to insulate the user. Rather, this compressed portion of the bags acts primarily to ensure continuity of the tubular portion of the bag, thereby preventing the easy escape of warm air within the bag or the infiltration of cold external air into the bag.
Within the last ten years or so, several companies have begun to sell backpacking quilts (also referred to herein as “quilt-style sleeping bags”) that eliminate most of the bottom portion of the bag that provides little insulating value. Backpacking quilts typically differ from traditional quilted blankets in that an enclosed or closable foot box is usually provided at a bottom end to ensure maximum insulation of the feet as well as minimize the intrusion of cold drafts of air. A user places his feet in the foot box and simply drapes the remainder of the quilt style bag over himself with the respective left and right edges extending over left and right edges of a corresponding sleeping pad not unlike the way in which a typical blanket is used when sleeping in a bed.
The advantages of a quilt-style sleeping bag over a traditional sleeping bag are several. First and perhaps the most important advantage to backpackers is that the weight of the bag is reduced, thereby lightening the load a backpacker will have to carry. Another advantage is that because quilt-style bags do not have zippers and typically use less material than traditional bags, they compress more than traditional bags, thereby taking up less volume in a user's pack. Further, since there is no zipper, zipper failure is not a concern.
There are, however, several disadvantages of using a quilt-style sleeping bag over a traditional sleeping bag. First for users who toss and turn or otherwise move significant amounts during sleep, there is a risk that they will push their bags off of their bodies exposing their bodies to cold air but also allowing any warmed air in the interior volume formed collectively by the bag and the sleeping pad to escape. Second and perhaps more significantly, there is a high likelihood that for even users that do not move too much during sleep, cold air will seep into the interior volume formed collectively by the bag and the sleeping pad as the left and right edges of the bag are occasionally raised above the edges of the associated sleeping pad when a user moves around even minimally during sleep.
Manufacturers of quilt-style bags have offered several solutions to resolve one or both of the disadvantages provided above. At least one manufacturer has provided one or more sets of straps 105 that extend from the left longitudinal edge 110 of the bag 100 to the right longitudinal edge 115 of the bag. In use, the straps are placed underneath the body of a user 120 as shown in Prior Art FIG. 1. While this solution does ameliorate some of the problems caused by the first disadvantage, the user's back (assuming the user sleeps on his/her back) will be exposed to cold air if he/she changes position during sleep.
Another solution offered by at least one manufacturer is to provide longer sets of straps 125 that are similarly attached to left and right edges 110 & 115 of the bag 100 such that the straps can be looped under a sleeping pad 130 as shown in Prior Art FIG. 2. Of particular note, the edges of the bag are drawn either up against the edge of the pad or underneath the pad. This solution solves the problems associated with the indicated disadvantages but introduces another problem. Namely, it creates a large interior volume 135 of air that must be warmed by the user's body heat. Large interior volumes of air space lower the effective temperature rating of a bag relative to a similar bag having the same amount of fill but a lower volume. Accordingly, to achieve a given temperature rating even more fill must be utilized raising the weight of the bag and the cost to produce it.